Impact of single-mask hole asymmetry on the properties of GEM detectors

A single-mask Gas Electron Multiplier (GEM) technique overcomes the cumbersome practice of alignment of two masks and allows the production of foils with very large area as needed for the CMS muon forward region upgrade. However, the holes obtained with refinements in the single-mask technique are asymmetrically biconical in shape compared to symmetrically bi-conical holes of double-mask technology. The hole geometry and their uniformity define the performance of the detectors which are constructed with such GEM foils. To evaluate the effect of this asymmetry, the foils have been characterized experimentally using a special prototype with three single-mask GEM foils. The structure allowed to change the orientation of foils, testing from above with foils having a large hole opening, testing from the bottom with all the foils having small hole opening. The effective gain, energy resolution and the charging up behavior are compared for the two different hole orientations.